The proposed projects will determine the behavioral mechanisms of action of methadone, examine the extent to which chronic maintenance on methadone interferes with ongoing behavior, and investigate the interaction of the behavioral actions of methadone with those of other psychopharmacological agents. These investigations will include determination of the modification of methadone's actions by stimulus control variables, the effects of methadone on performance controlled by temporal stimuli, the effects of schedule variables on the actions of methadone, and the effects of "motivational" variables on the behavioral effects of methadone. Once these baseline phenomena have been investigated, the interactions between the effects of acute and chronic methadone and those of acutely administered ethanol, tetrahydrocannabinol, pentobarbital, and cocaine will be determined. Finally, this project will investigate optimal methods for long term administration of methadone to laboratory subjects.